Critics have called the Electoral College antidemocratic after the College decided two of the past five presidential elections. George W. Bush and Donald Trump both lost the national popular vote in their respective elections in 2000 and 2016, but went on to become president.

On Tuesday, Senate Democrats, led by Hawaii Sen. Brian Schatz, and with support from Democratic Sens. Dick Durbin of Illinois, Dianne Feinstein of California and Kirsten Gillibrand of New York—also a 2020 presidential contender—introduced a constitutional amendment to do away with the institution altogether.

But why do they want to abolish it? Here’s what you need to know.

What Is the Electoral College?

The Electoral College dates back to the 1787 Constitutional Convention in Philadelphia, where a body of electors was formed. This body would have the final say on the country’s leadership.

Every presidential election, candidates campaign in a state-by-state race, not only to win the most votes, but also to win their respective electoral votes. The number of electors varies by state: Alabama, for example, has nine; Florida has 29; Massachusetts has 11; Vermont has three. The power to determine the president of the United States is ultimately reserved for the 538 electors, as candidates race to win at least 270 electoral votes in the general election.

Most states award electoral votes on a winner-takes-all basis, meaning that the candidate to win the most votes in a given state will take all of that state’s electoral votes, as well. Nebraska and Maine are the only two states that do not follow the winner-takes-all rule. Electoral votes are instead allocated proportionally.

In the 2016 race, Trump won in many of the Southern states and across parts of the Midwest, awarding him 304 electoral votes, compared to Hillary Clinton’s 227, winning him the election. Despite Trump’s Electoral College victory, Clinton won nearly 3 million more nationwide votes than Trump.

The Movement to Abolish the Electoral College

The movement to abolish the Electoral College has become popular in the past two years. Top Democrats are speaking out against the political system as the country prepares for the 2020 election, some rallying behind the idea of one person, one vote.

During a CNN town hall last month, Massachusetts Sen. Elizabeth Warren endorsed eliminating the Electoral College. “My view is that every vote matters. And that means get rid of the Electoral College,” she said.

Pete Buttigieg told The Washington Post, “It’s gotta go. We need a national popular vote.” Beto O’Rourke said “there’s a lot of wisdom” in eliminating the Electoral College, and Sen. Kamala D. Harris told Jimmy Kimmel she’s “open to the discussion.”

When reached for comment, Miramar, Fla., Mayor Wayne Messam called the Electoral College “an imperfect attempt to provide representation for the states.” Messam added: “America should be a place where every vote is encouraged and counted and our election results should reflect the will of the American People, no matter which state they reside.”

Others are less invested in the issue. Former Maryland Rep. John Delaney who announced a 2020 run called it “a hypothetical debate about something we all know is not going to happen.” Instead, Delaney suggested focusing on “expanding access to healthcare, passing an equal rights amendment, and rebuilding our outdated roads and bridges.”

But candidates are only campaigning in a handful of states. According to data compiled by the non-profit corporation, National Popular Vote, 94% of political events in 2016 took place in just 12 states, 11 of which were deemed “battleground” states (Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin), plus Arizona.

In 2020, there could be even fewer battleground states, Dr. John Koza, the chair of the National Popular Vote, told Fortune. According to Koza, this could pose a problem moving forward as just five or six battleground states hold more power in determining where 2020 campaign efforts are focused.

Koza’s organization is working to change the winner-takes-all system in favor of an interstate compact that will allow states to give their electoral votes to the presidential candidate who wins the most national votes. So far, 14 states have signed on to the agreement, and hold a combined 184 electoral votes. The agreement can be enacted once there are a combined 270 electoral votes between the participating states to allocate to the presidential candidate with the most national votes.

“The most important problem to us is because of [the winner-takes-all system], candidates only campaign in a handful of states,” Koza said. He added that presidential candidates and sitting presidents alike have made promises to enact “policies based on what battleground states they want to win.”

Most Americans prefer the popular vote, as well. A recent poll from the Public Religion Research Institute found that 65% of Americans said presidential elections should be decided by the national popular vote, while 32% said the Electoral College should decide.

Some top Republicans disagree.

The Case for Keeping the Electoral College

“The desire to abolish the Electoral College is driven by the idea Democrats want rural America to go away politically,” Senator Lindsey Graham of South Carolina said on Twitter.

Some Republicans say that smaller states and more rural states will have less influence in elections that rely only on the popular vote. But these states already have little power under the current system, which relies heavily on winning over a small number of deciding states.

Other defenders have argued that doing away with the Electoral College would put white Americans at a disadvantage, a far-right talking point. Former Maine Gov. Paul LePage argued along these lines earlier this year, and suggested big states like California, Florida, and Texas would have more power in deciding the president. “It’s only going to be the minorities that would elect,” he said in an interview.

Support for changing the rules around the Electoral College has shifted after various elections. Republican states showed more interest in the National Popular Vote interstate compact after former President Barack Obama’s 2008 win, for example.

Some states have shown bipartisan support for the measure, though. Lawmakers in Michigan—a battleground state in 2016 and likely again in 2020—introduced a bipartisan National Popular Vote bill last year.

Trump has changed his position on the matter.

In 2012, Trump called the Electoral College a “disaster for a democracy.” He later amended his stance after winning in 2016, arguing that smaller states and the Midwest “would end up losing all power” if the Electoral College were eliminated.

But the logic is flawed, said Koza, who called these positions “political rhetoric and it’s not connected to fact.”

“Well the rural states don’t gain anything from the current system,” said Koza. “The top 10 rural states—none of them are battleground states, so the notion that rural states somehow benefit [from the Electoral College] is nonsense.”